Most modern transducers for use in diagnostic medical ultrasound imaging are phased arrays, made up of single or multiple rows of electrically and mechanically independent transducer elements. Each transducer element is a layered structure comprising an acoustic absorber, a piezoelectric ceramic (hereinafter "piezoceramic") layer, one or more acoustic matching layers, and a front wear plate or focusing lens. Typically, one or more flexible printed circuits (hereinafter "flex circuits") are used to make electrical connections (signal and ground) from the piezoceramic layer to the signal processing electronics, or to a bundle of coaxial cables which ultimately connect to the signal processing electronics. One known method of connecting the flex circuit to the piezoceramic uses ohmic contacts; i.e., exposed metal pads on the flex circuit are laminated, using high pressure and a thin layer of non-conductive epoxy, to the electroded surface of the piezoceramic layer. If the flex circuit and piezoceramic surfaces are microscopically rough and the epoxy layer is sufficiently thin, then an electrical connection is achieved via a distribution of direct contacts between high points on the piezoceramic surface and high points on the flex circuit.
The quality of such an ohmic electrical connection is very sensitive to material and process parameters that can be difficult to control (such as surface roughness, flatness, and parallelism; epoxy viscosity; lamination pressure). The resistances of both good and bad contacts are small compared to other impedances in the circuit for a transducer element, so that it is next to impossible to non-destructively measure the quality of the electrical connection. Typical measurements of transducer performance, such as low-frequency capacitance measurements or high-frequency impulse response measurements, can detect open or severely degraded ohmic contacts but cannot discriminate between good contacts and weak contacts which may degrade and become unreliable over time. If contact problems are suspected, either the back or front layers of the transducer must be removed to obtain access to the flex circuit-to-ceramic bond area. The ceramic electrode and the flex circuit metallization are separately exposed and connected to the leads of a resistance meter. This method of analysis is both destructive and laborious.